Pulse length modulation system



Dec. 27, 1949 TRIGGER INPUT T. G. CUSTIN 2,492,736

PULSE LENGTH MODULATION SYSTEM Filed Feb. 26, 1949 F' igl PULSE OUTPUTTZZ lNTEGRATED PULSE TERMINATING PULSE TIME Invent r" Thomas G. Custin yzzmza 0% His Attorny Patented Dec. 27, 1949 PULSE LENGTH MODULATION SYSTEM Thomas G. Custin, Syracuse, N. Y., assignmto New York General Electric Company. a corporation of Application February 26, 1949, Serial No. vacos 2 Claims. (01. sea-14) This invention relates to pulse length modulation systems generally, and more particularly to means for generating pulses and for varying the length or time-duration of these pulses in accordance with a modulating signal.

An object of the present invention is to provide a new and improved system for generating pulses of voltage of constant amplitude whose length can be varied in accordance with a signal modulation.

Another object of this invention is to provide a pulse modulation system in theform of an electronic multi-vibrator circuit in which the cuit in which device I is normally conducting initiation of a pulse is determined by an input triggering voltage, and its termination is determined by the instantaneous amplitude of a modulating signal.

For further objects and advantages and for a better understanding of the invention, attention is now directed to the following description and accompanying drawing, and also to the appended claims in which the features of the invention believed to be novel are particularly pointed out.

In the drawing:

Fig. 1 is a schematic circuit of a pulse length modulating system embodying my invention.

Fig. 2 shows a series of curves on a common time scale, graphically illustrating diflerent voltage waveforms appearing in difierent parts of the system of Fig. 1 during the occurrence of a pulse.

Referring to Fig. 1, there is shown an electronic multivibrator circuit comprising a pair of elec-- tronic discharge devices I and 2. Device l comprises an anode 3, a cathode 4, and a control grid 5; and similarly device 2 comprises an anode 6, a cathode l, and a control grid 8. These devices are connected in a manner well known to the art to form a multivibrator having one state of stability. The anode 3 of device I is coupled to the grid 8 of device 2 through a series combination of a capacitor 9 and a resistor l0, grid 8 having in addition a connection to ground through a resistor II. The anode 6 of device 2 is connected to the grid 5 of device 3 through a capacitor l2, grid 5 having in addition a connection to a source of operating potential B+, through a resistor I3. The cathodes of both devices are connected to ground through a resistor ll which is shunted by a capacitor l5. Anode voltage for both devices is provided through resistors l6 and II, respectively, both being connected to the source 3+.

Devices I and 2, with the circuit as thus far described, constitute a simple multivibrator cirand device 2 is normally non-conducting. This is due to the fact that a positive voltage tends to develop at the grid 5 of device I due to its connection to the source of positive potential through the resistor l3, while a negative potential tends to develop at the grid 8 of device 2 due to its connection of grid 8 to ground through resistor ll. Also, a bias is developed .by the flow of current from device I, through the cathode resistor ll. In the absence of any external voltages disturbing it, this static condition of current through device I and absence of current through device 2 would continue indefinitely.

The remainder of the circuit to be described is for the purpose of disturbing or upsetting this static state of the multivibrator by reversing the operating condition of the devices from their previously conducting and non-conducting states, to non-conducting and conducting states respectively, for a finite time interval determined in accordance with a modulating signal.

A trigger input terminal 20 is connected to the anode 3 of device I through a capacitor 2|, and a pulse output terminal 22 is likewise connected to anode 3 through a coupling capacitor 23. Anode 3 is moreover connected through a capacitor 24 and a resistor 25 to the control grid 26 of an electron discharge device 21. Device 21 has an anode 28 connected to the source of operating potential through a resistor 29, and a cathode 30 connected to ground through a resistor 3| shunted by a by-pass capacitor 32.

Cathode 30 is connected to the cathode 33 of an electron discharge device 34. Device 33 has an anode 35 connected to the source of operating potential B+ and a control grid 36 connected through a resistor 31 to a tap 38 on a potentiometer 39. This potentiometer is connected between the source of operating potential and ground. Grid 36 is connected through a coupling capacitor an to a source of modulating signal voltage ll. The anode 28 of device 21 is coupled through a capacitor 42 to the grid 8 of device 2. The control grid 25 of device 21 is connected to ground through a parallel combination of resistor 43 and a capacitor 44.

In operation, a positive triggering pulse of voltage is applied to terminal 20 and is coupled therefrom to the anode 3 of device I and to the.

grid 8 of device 2. The positive voltage at the grid of device 2 starts conduction of current through that device. thereby developing an initially small negative increment of voltage at its anode which is coupled back to the grid 5 of device I through capacitor I2. The negative is non-conducting. This is due to the fact that negative charge existing across capacitor I 2 is slowly dissipated through resistor I3, while the positive charge existing across capacitor 9 is dissipated through resistor II. The time duration during which the unstable condition of conductioniexists is largelydependent upon the time constant of the combination of resistor l3 and capacitor I2. However, the unstable condition of conduction can be terminated by applying a pulse having a polarity opposite to that of the original pulse, that is a negative pulse, to the grid 8 of device, 2. This pulse is coupled from the anode 28 of device 21, through capacitor 42, to the grid 8 of device 2. The time interval at which this terminating or shut-oft pulse is applied is determined by the initiation of conduction through device .21.

Normally, device 21 is maintained in a nonconducting condition by the negative bias resulting at its grid from the positive voltage developed at its cathode 30 through the conduction of current through resistor 3| and device 34. .Device 34 is always conducting and an average current flows through it in accordance with the average voltage applied to its grid, as determined by the setting of tap 38 on potentiometer 39. The instantaneous current flowing through device 34 is determined by the modulating signal coupled to grid 36, through capacitor :38, from the source of modulating signal ii i The current through resistor 3i then has an instantaneous amplitude in accordance with the amplitude of the modulating signal, and the voltage existing between grid 25' and ground varies accordingly.

Beginning with the initiation of an unstable condition of conduction through devices i and 2, the positive voltage at the anode 3 of device 2 causes capacitor M to begin to charge through resistor 25 which is connected to it through a comparatively large coupling capacitor 25. Resistor 25 and capacitor 3 3 operate as an integrating circuit to produce a saw-tooth voltage. When the voltage developed across capacitor dd has reached a sufilciently large value to overcome the negative bias at grid 2t, conduction starts in device 2? and, accrdingly, the voltage at the anode 28 decreases sharply and applies a negative pulse to grid 8 of device 2. This in turn causes an increment of positive voltage to be produced at the anode t of device 2 which initiates a cycle of changes in reverse direction in the multivibrator circuit, which is terminated by a reversion to the original condition of stability. Thus the negative pulse at anode 28 serves as a shut-oil pulse to terminate the duration of the increment in voltage at the anode of device 1! of the multivabrator circuit. At any instant of time, the negative voltage existing between grid 26 and ground, due to the current flowing through resistor Si, is dependent on the instantaneous amplitude oi the modulating signal. Thus, the voltage across capacitor is has'to attain agreater or lesser magnitude, dependent upon the instantaneous amplitude of the modulating signal, before conduction is initiated 'in device 21. The termination of the multivibrator pulse occurs when the negative bias at grid 28 and the sawtooth voltage coincides in amplitude. Since capacitor ll charges at a substantially linear rate. the time interval between the initiation of the multivibrator pulse at the anode of device I and its termination thus varies in accordance with the instantaneous amplitude of the modulating signa Referring to Fig. 2, the operation of the circuit may more readily be understood through a consideration of the waveforms shown therein. Curve 5! indicates the positive triggering pulse applied to terminal 2| at a time'to, Curves 5| and 52 indicate the voltages developed at the anodes of devices I and 2, respectively, the solid line being for one instantaneous amplitude of modulating signal, and the .dotted part of the curve being for a greater instantaneous amplitude of modulating signal. The leading edges of the multivibrator pulses occur, for all practical purposes, substantially at the same instant to, as the occurrence of the leading edge of the trigger pulse. Curve 52 indicates the saw-tooth voltage developed across capacitor 44 as a result of its charging through resistor 25 from the voltage existing at the anode of device 1, as represented by curve 5|. By choosing resistor 25 and capacitor 44 such that the time constant of the combination is appreciably longer than the maximum desired length of time-duration of the output pulses, capacitor l4 charges substantially linearly during the time interval defined by the maximum time-duration of the pulses. For purposes of illustration, the maximum time duration of the pulses has been indicated at t2 and capacitor 64 charges substantially linearly up to that time. In the curves shown, the modulating signal has an amplitude such that device 2'! begins to conduct when capacitor 44 has charged to an amplitude A as indicated with reference to curve 53. This amplitude is achieved at a time h and,

accordingly, device 21.begins to conduct at that instant and a negative shut-oil? pulse is coupled to the grid s of device 2 as indicated by curve as, which terminates the multivibrator pulse. The time t; at which the multivibrator pulse is terminated can be varied up to time is, while still maintaining substantial linearity between the instantaneous amplitude of the modulating signal and the len th or duration of the pulse.

In a practical construction of an embodiment of my invention, the following values of circuit elements have been found to provide emcient operation:

A circuit embodying my invention and utilizing the above mentioned values of elements was found on experimental test to provide very linear modulation. The repetition rate was fixed at 12,000 cycles per second and the pulse length was varied from a minimumof 0.5 microsecond to a maximum of 3.5 microseconds. In the frequency band from 100 to 3,500 cycles, the experimentally observed linearity of modulation was better than 98 percent when the pulse length was modulated to '75 percent of its mean length of 2 microseconds. Stated in other words, the pulse length was set at 2 microseconds in the absence ofmodulation, and modulation was introduced to vary the pulse length by plus or minus 1.5 microseconds. Throughout this range of modulation, the pulse length was found to vary substantially linearly with the instantaneous amplitude of the modulating signal, the deviation from a linear relation not exceeding 2 percent.

While a specific embodiment has been shown and described, it will, of course, be understood that various modifications may be made without departing from the invention. The appended claims are, therefore, intended to cover any such modifications within the true spirit and scope of the invention.

What I claim as'new and desire to secure by Letters Patent of the United States is:

l. A system for generating output pulses, comprising a multivibrator having one condition of stability, means to reverse said condition of stability and initiate a condition of instability whereby an increment of voltage is produced in said multivibrator, an integrating circuit, said increment being applied to said circuit to produce a saw-tooth voltage, a discharge device, means to produce a control potential varying in accordance with the instantaneous amplitude of a modulating signal, means to oppositely apply said control potential and said saw-tooth voltage to said device to prevent conduction through said device until said saw-tooth voltage coincides in amplitude with said control potential, said coincidence producing a shut-oil pulse in said device, and means to apply said shut-off pulse to said multivibrator to end said condition of instability and restore said condition of stability thereby eliminating said increment of voltage, the duration of said increment providing said output pulse.

2. A system for generating output pulses having a time-duration proportional to the instantaneous amplitude of a modulating signal, comprising a multivibrator having one condition of stability, means to reverse said condition of stability and initiate a condition of instability, whereby an increment of voltage is produced in said multivibrator, an integrating circuit, said increment being applied to said circuit to produce a saw-tooth voltage rising linearly with time, a discharge device having a cathode, an anode, and a grid, means to apply a control potential to said grid to maintain said device in a normally non-conducting condition, means to vary said potential in accordance with the instantaneous amplitude of said modulating signal, said sawtooth voltage being applied to said grid in opposition to said control potential whereby said device conducts and generates a shut-off pulse at its anode after a time interval determined by the coincidence in amplitude of said saw-tooth voltage with said control potential, and a connection for applying said shut-oil pulse to said multivibrator to restore said condition of stability, thereby eliminating said increment of voltage, the duration of saidincrement providing said output pulse.

THOMAS G. CUSTIN.

lilo references cited. 

